The present invention generally relates to a method of bonding a semiconductor die to a package and more particularly to a direct gold/silicon eutectic bonding method.
Conventional die bonding methods include a wide choice of materials that are used in the attachment of silicon dies (with or without back metal) to substrates. These materials include hard and soft solders, conductive epoxies, and gold/silicon eutectic alloys. Gold/silicon eutectic alloys are the preferred choice for small packages such as SOT-23 and SOT-363 and are the only choice for even smaller packages such as SOT-563 and SOT-623.
Conventional gold/silicon eutectic die bonding methods include coating the back surface of the silicon die with gold and then heating the die to a temperature above the eutectic temperature of 370° C. to form the gold/silicon eutectic alloy. In most applications, an adhesion layer of sandwich type structure is used to improve the adhesion of the gold to the silicon die. The die is then attached to a metal leadframe or substrate by an additional heating step which utilizes the gold/silicon eutectic alloy as a solder. Additional layers of gold may be deposited over the gold/silicon eutectic alloy layer and gold or gold alloy pre-forms may be used to facilitate die bonding. Other conventional techniques include using a gold/silicon seed as a catalyst for forming the eutectic bond and using a gold/silicon alloy produced by silicon implantation in a gold layer.
Increased performance requirements of vertical power device products are driving manufacturers to integrate and assemble these devices using thinner wafer processes. Prior art methods of coating the back surface of silicon die formed on thin wafers with metal present difficulties. There is therefore a need for a die bonding method that is simple and does not require a back metal process. There is also a need for a die bonding method that does not require a pre-form, eutectic solder, or seed implant. There is a further need for a die bonding method that is cost efficient and that increases device throughput.